What to do with twins

Now that the boys are older, I'm finding I have more twin related issues to deal with when it comes to the social scene. The boys are in different classes and as a result, birthday party invites will come in the mail for one boy and not the other. Moms will call to schedule a play date with one of the boys, not realizing that he has a twin brother. Once I explain the situation to the mom, I always feel a little bad for disrupting the original plan (to just have one of the boys over). All the moms have so far been very understanding and willing to let the other twin tag along. I suppose it works OK. At this age, it's just too hard explaining to the one unintentionally left behind why he can't go play too. I suppose it isn't really a big deal, but at the same time, I wonder- at what point do I let them develop their own friendships? If they're joined at the hip for all events, when do they learn to grow independently of each other? I'm not sure I know the answer to this one right now. Maybe I'll start by asking them what they want to do and then figure each situation out as I go.